A boiling water reactor is typically equipped with an external circulation system for forcefully feeding coolant into a reactor core. A plurality of sets of jet pumps connected to the external circulation system are arranged in an annular downcomer portion between a core shroud and a reactor pressure vessel.
A jet pump mainly includes a riser, a diffuser, and an inlet mixer. To install the jet pump in the reactor pressure vessel, the riser, the diffuser, and the inlet mixer are installed in such order. The diffuser is welded and mounted onto a baffle plate in the reactor pressure vessel.
The inlet mixer is installed by passing a lower end of the inlet mixer through a riser bracket attached to a side surface of the riser pipe, and then inserting the lower end into an opening in the top of the diffuser. A seat surface of an elbow part of the inlet mixer is placed on the top of the riser. A head bolt of a jet pump beam for installing the inlet mixer is then tightened to fix the inlet mixer to the upper portion of the riser.
After the head bolt is tightened to install the inlet mixer on the top of the riser via the jet pump beam, the head bolt threadedly coupled with the jet pump beam needs to be locked against rotation.
Among conventional bolt fixing devices for a jet pump beam there is a technique, for example, described in Patent Document 1. The technique includes providing an axially-slidable, integrally-rotating lock cap on a polygonal head portion of the head bolt, and arranging a body housing accommodating the lock cap on an upper surface of the jet pump beam. A plurality of tapered external teeth are formed on an outer peripheral side surface of the lock cap. The body housing has internal tooth-like grooves to selectively mesh with the external teeth of the lock cap. The external teeth of the lock cap and the internal tooth-like grooves are meshed with each other, whereby the head bolt of the jet pump beam is fixed.